1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to reproducing information using a field effect semiconductor probe, and more particularly, to reproducing information in which a thermal signal generated by the surface structure of media and an information signal generated by a variation in an electric field of the media can be separated from each other.
2. Description of the Related Art
As the demand for small-sized products such as portable communication terminals and electronic pocketbooks increases, highly-integrated micro nonvolatile recording media are increasingly required. It is not easy to reduce the size of existing hard disks and to highly integrate flash memories at low cost. Thus, an information storage media and a method using a scanning probe have been studied as a possible alternative.
The scanning probe is used in various types of scanning probe microscopes (SPMs). For example, the scanning probe is used in a scanning tunneling microscope (STM), an atomic force microscope (AFM), a magnetic force microscope (MFM), a scanning near-field optical microscope (SNOM), an electrostatic force microscope (EFM), and the like. The STM detects a current flowing through a probe based on a difference between voltage applied to the probe and a sample to reproduce information. The AFM uses an atomic force between a probe and a sample. The MFM uses a magnetic force between a magnetic field near the surface of a sample and a magnetized probe. The SNOM has an improved resolution less than the wavelength of visible light. The EFM uses an electrostatic force between a sample and a probe.
Lim et al. suggests a field effect probe for detecting a surface charge of media using a field effect (see U.S. Pat. No. 6,521,921 and Korean Patent No. 0366701). The probe suggested by Lim et al. has a semiconductor tip in a field effect transistor shape in which a carrier channel is formed by an electric field. The electric field applied to the semiconductor tip is formed by charges or a dipole moment trapped on the surface of the media. When the charge trapped on the disk corresponding to written information forms an electric field having strength greater than a threshold field strength, a channel is formed and the resistance of a field effect probe is reduced. Thus, information written using a variation in a resistance corresponding to the written information can be reproduced.
In addition, Park et al. who are also the inventors of the present application suggest a resistive semiconductor probe in which a channel region of the semiconductor tip is lightly doped (see U.S. Patent Application Publication No. 2005/0231225 A1). The semiconductor tip of the resistive semiconductor probe is lightly doped with impurities so that a weak current can flow even without an electric field present. Thus, the semiconductor tip can be detected even in a weak electric field. That is, a method suggested by Park et al. guarantees high sensitivity to a charge even in a weak electric field by providing low electron mobility in which a carrier moves even in a non-electric field to the semiconductor tip.
However, the resistive semiconductor tip is sensitive to heat. Thus, a resistance thereof varies according to temperature. A variation in a resistance caused by thermal instability appears as a defect of the resistive semiconductor probe. That is, a variation in unstable temperature of the probe causes an unstable current variation which is a noise current in the resistive semiconductor tip. The noise current is generated by a variation in temperature regardless of an electric field. An unstable temperature variation in the resistive semiconductor probe is generated because heat generated in a probe or a cantilever supporting the probe is not uniformly and continuously dissipated by an unstable variation in a distance between media and a probe or a contact area.
In order to suppress an unstable variation in the temperature of the resistive semiconductor probe, the distance between the probe and the media should be maintained. To this end, the surface of the media that faces the probe may be made to be very smooth. Even though the smoothness of the surface of the media is maximized, it is not possible to obtain sufficient and effective thermal stability due to a limit of achievable smoothness. This is because, even when the distance between the media and the probe varies within the range of several nm, a noise current is generated by a variation in temperature and even when the surface of the media is processed to be smooth, like a mirror, the smoothness of the surface of the media cannot be adjusted within the range of several nm. As another alternative, the distance between the probe and the media may be sufficiently large. The feasibility of the alternative is low since it is difficult to manufacture a resistive semiconductor probe having a high aspect ratio. There is almost no possibility of a noise current caused by thermal instability being entirely eliminated even in the semiconductor probe suggested by Lim et al.
Accordingly, a method for effectively reproducing a signal in spite of a noise current caused by thermal instability of a semiconductor probe by improving a signal-to-noise (S/N) ratio is desired, so as to effectively read information from media on which information is written using a charge, using a semiconductor probe in which the flow of current is controlled by a field effect.
An information writing/reproducing method by which the information is represented by a variation in the shape of the surface of a disk has been suggested (William P. King et al., Volume 78, Number 9, Applied Physics letters, 26 Feb. 2001). In this technology, a data bit is formed on a thin film on a media by thermally melting the surface of the disk using a probe in order to write information on the media. When information is reproduced from the media, a variation in the resistance of or current in a cantilever is detected using a variation in the amount of heat dissipation according to a variation in a distance between the cantilever and the surface of the media due to a variation in the shape of a substrate which represents the bits of information.
One aspect of development of storage technology is to maximize the density of written information. To this end, the development of new media for maximizing the density of written information is required and an apparatus for writing/reproducing information that can support the development of the new media is also needed.